PBX, MEIS/PREP, and HOX genes encode interacting transcription factors that regulate development. Their mutation produces oncogenes that cause human leukemia and other tumors. Pbx, Meis/Prep, and Hox proteins are also implicated as subordinate oncoproteins functionally required for leukemogenesis by other human oncogenes. Thus, understanding oncogenesis by individual Pbx, Meis/Prep or Hox oncogenes will yield a broader understanding of human leukemia in general. This proposal focuses on understanding how Pbx proteins control development and hematopoiesis (Aims 1 and 2) and how the human E2a-Pbxl oncogene causes AML and preB ALL (Aims 3 and 4). Health-relatedness: Just as understanding oncogenesis by signal transduction oncoproteins led to the development of therapeutic inhibitors of cell proliferation, so also understanding the biochemical mechanism of oncoproteins that block hematopoietic differentiation will establish a rational to generate drugs that inhibit their function and promote differentiation. Because oncogenes that promote proliferation and inhibit differentiation cooperate to cause leukemia, drugs that inhibit proliferation and promote differentiation should cooperate to cure leukemia. Aim 1. Determine how Pbx homodimerization (in the absence of DNA) and Pbx:Meis/Prep heterodimerization regulate DNA-binding and nuclear import of Pbx proteins. Determine how Pbx proteins function as transcriptional coactivators. Aim 2. Using knockout technology, determine the role of Pbx2 in mouse development in general, and in regulating lineage commitment, differentiation progression, and gene transcription during hematopoiesis in particular. Make conditional Pbx2/Pbxl, or Pbx2/Pbx3 mice to determine the effect of double knockouts on hematopoietic differentiation. Aim 3: Use conditional E2a-ER-Pbxl to determine how E2 about-Pbxl prevents transcription of myeloid differentiation genes, focusing, mechanistically, on MRP8. Determine whether direct E2a-Pbxl targets reestablish a differentiation block in conditionally-immortalized E2a-ER-Pbxl myeloblasts and are activated in human t(l;19) pre-B ALL (L-Myc is one such target). Aim 4: Establish the cellular impact of E2a-Pbxl in models of pre-B cell leukemia induced by coexpression of Ras6l L plus E2a-Pbxl, or in transgenic mice expressing wild-type or conditional E2a-ER-Pbxl genes driven by the early, hematopoietic specific, Vav promoter. Identify the biochemical domains of E2a-Pbxl required to cause pre-B ALL, and begin to identify a genetic mechanism by which E2a-ER-Pbxl causes pre-B cell ALL.